The neurotransmitter dopamine is significant in the control of sexual behavior in male and female mammals. In female mice, dopamine facilitates lordosis through the ligandindependent activation of the progesterone receptor. The progesterone receptor is involved in female receptivity and male mounting behavior in rats and mice and also facilitates male-like pseudosexual behavior in a unisexual species of lizard, Cnemidophorus uniparens. Using normal and transgenic (progesterone receptor knockout) mice I propose to investigate interactions between dopamine and progesterone receptor, and how those interactions affect masculine sexual behavior. Transgenic mice represent a unique model system whose study will enable a better understanding of the neural substrate underlying male sexual behavior. These experiments will also provide training in endocrine physiology, immunocytochemistry, and behavioral testing techniques.